A vacuum-assisted, countergravity casting process using a gas permeable mold sealingly received in a vacuum housing is described in the Chandley et al U.S. Pat. No. 4,340,108. That countergravity casting process involves providing a mold having a porous, gas permeable upper mold member (cope) and a lower mold member (drag) sealingly engaged at a parting plane, sealing the mouth of a vacuum housing to a surface of the mold such that a vacuum chamber confronts the gas permeable upper mold member, immersing the underside of the lower mold member in an underlying molten metal pool and evacuating the vacuum chamber to urge the molten metal upwardly through one or more ingates (also referred to as pin gates) in the lower mold member into one or more mold cavities formed between the upper and lower mold members.
The ingates are so sized (e.g., typically less than 0.25 inches in diameter) to effect relatively rapid initial solidification of the molten metal in the ingate(s) prior to substantial solidification of the molten metal in the mold cavity. Once a plug of the metal is solidified in the ingate(s), the metal-filled mold can be withdrawn from the underlying pool without molten metal run-out from the mold cavity. The time that the mold ingates must remain immersed in the pool is thereby reduced and shortens the casting cycle time.
Since mold ingate immersion times in the molten metal pool (e.g., an iron melt) are on the order of only about 15 to 20 seconds using this process, the reduction of even a few seconds in the mold immersion time is appreciable and results in increased production of cast parts at lower cost.
Accordingly, it is an object of the present invention to provide an apparatus and method for the differential pressure, countergravity casting of molten metal wherein the time required for engagement between the mold ingate(s) and an underlying molten metal source is reduced to shorten casting cycle time and improve production economies.
It is another object of the present invention to provide an apparatus and method for the differential pressure, countergravity casting of molten metal wherein a chill member is disposed in the mold ingate(s) to accelerate solidification of a plug of the metal therein after the mold cavity is filled with the melt and before the mold ingate(s) and the molten metal source are disengaged.